The present invention relates to microwave oscillator and mixer combinations, and particularly to microwave receivers.
Many microwave receiver systems use a balanced or doubly balanced mixer to achieve adequate LO-to-RF isolation. In a balanced mixer, the local oscillator provides two inputs having the same frequency but different phases. A diode ring mixer can then achieve mixing with good isolation.
The conventional balanced mixer uses either a 90 degree or a 180 degree transformer to generate the necessary balanced inputs; at microwave frequencies, this transformer is typically realized by use of hybrid couplers (for a 90 degree transformer) or balun structures (for a 180 degree transformer). In either case, the phase and amplitude of the transformer output do not remain constant over frequency; this frequency dependence causes degradation of harmonic suppression, and also causes the LO-to-RF isolation to be dependent upon diode match.
A 180 degree mixer has good isolation characteristics if the diodes are balanced, regardless of the goodness of the impedance match. Phase errors are multiplied by the order of the LO harmonic, and therefore very large phase errors in conventional schemes can be expected. Typical balun phase errors can easily be 50 to 10 degress over bandwidths of less than an octave.
As the bandwidth of the conventional structure is sought to be increased, the requirements for the layout become more complex and less producible. Suspended substrate and fin-line structures are common methods to achieve the coupling and impedance levels required. These structures are difficult to implement, and, in the case of suspended substrate, mechanical integrity with temperature variation is always a problem.
The present invention is highly advantageous in permitting mixers to be integrated on GaAs (or other III-V substrate). Since monolithic oscillators, low noise amplifiers, and transmit-receive switches have already been implemented on GaAs, the only circuits preventing integration of a complete down conversion front-end is a balun and an active mixer topology. The present invention allows the oscillator and mixer to be directly interfaced, which will eliminate the need for a balun structure and will reduce the chip area required for a mixer/oscillator chip.
Of course, other mixer designs besides a diode ring can be used. For example, a differential pair of MESFETs can be used, or one or more dual-gate FETs, or active-device configurations. These active-device configurations may in fact be more advantageous for integration on a single monolithic substrate.
According to the present invention there is provided: A microwave receiver comprising: a local oscillator comprising first and second field-effect transistors each comprising a gate and first and second source/drain terminals, and a resonant circuit connected between the gates of said first and second field-effect transistors, said gates of said first and second field-effect transistors being located at opposite-phase points of said resonant circuit; a balanced mixer having an rf input and a mixed output and a first local-oscillator input operatively connected to one of said source/drain terminals of said first field-effect transistor and a second local-oscillator input operatively connected to one of said source/drain terminals of said second field-effect transistor; means for providing an rf microwave signal to said balanced mixer; and means for filtering the output of said mixer to remove frequency components corresponding to said rf microwave signal therefrom.